This invention relates to proximity sensors and related circuits, and particularly to such systems adapted for sensing metal workpieces in hostile conditions.
Proximity sensors are used in various facets of manufacturing for detecting the approach of metal objects. An example of a suitable application for such devices is for automated sheet metal forming lines such as progressive die press operations where proximity sensors could be used for determining whether or not material handling systems are properly engaging workpieces as they are moved from one work station to another. If a part engaging member such as a shovel or articulated gripper does not properly receive a workpiece and such absence is detected, the material handling and fabrication machinery could be interrupted to correct the failure. Workpieces which are not in their proper position can lead to the generation of scrap and can also damage equipment of the fabrication system.
Inductive type proximity sensors have been in use for many years and operate on the principle that approaching magnetic objects change the inductive characteristics of the sensors, which, in a simplified form, is no more than a coil of wire wrapped around a ferrite core. This change in inductance characteristic produces a reduced output from a resonant tank circuit which the sensor is a part of. The tank circuit voltages are detected and output through appropriate signal processing electronics. Present proximity sensor systems operate at relatively high frequencies (e.g. 200 to 300 KHz), and are used with high "Q" tank circuits (i.e. circuits with high impedance to resistance ratios). Such sensor systems cannot sense through a layer of metal to detect objects on the opposite side of the layer, due to eddy current losses occurring in the metal layer. Accordingly, such proximity sensors have a sensing face which is nonmetallic. Typically, the coils are potted using epoxy compounds or other plastic materials which cover the sensing face. In many applications for proximity sensors such as the application discussed above, proximity sensors are exposed to extreme environmental conditions where they can be struck by metal workpieces and subjected to abrasives, cutting fluids, etc. For such applications, the vulnerable configuration of prior art proximity sensors renders them unsuited for use. Accordingly, there is a need to provide a proximity sensor which can be encased in a durable material such as a metal, while providing sensitivity for detecting other metal objects.
In addition to the foregoing, since present inductive proximity sensors are used with high "Q" tank circuits, and since a full metallic enclosure results inherently in a circuit with a low "Q" value, there is a need for an amplifier system which can function with a low "Q" circuit and detect small changes in that "Q" value.
In accordance with this invention, several embodiments of proximity sensor systems achieving the above mentioned desirable characteristics are provided. Proximity sensors according to this invention can be encased in metal such as stainless steel while enabling the detection of approaching metal objects. The resulting sensor is extremely durable and can therefore be used in hostile operating conditions. These capabilities are achieved, in part, by driving the proximity sensor coil at a relatively low frequency, for example, at less than 10 KHz. The stainless steel encasing material is relatively invisible to the low excitation frequency, since eddy current losses decrease with frequency. The reason that a portion of the sensing field extends through a given thickness of stainless steel is that the product of circuit "Q" and skin depth have been maximized. Skin depth is greater for a high resistivity material like stainless steel and it also is larger for lower frequencies. Therefore, the relative "invisibility" of the stainless is due to the low frequency used and to the high electrical resistivity of the stainless. These features make it necessary to employ relatively low "Q" tank circuits (since "Q" decreases with frequency). Various circuit designs are disclosed as means for evaluating small changes in output of the sensing inductor which provide excellent sensitivity, while enabling use of low "Q" low frequency sensing circuits.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.